Large format color ink jet printers generally have a printhead carriage which is mounted for reciprocal movement on the printer in a direction orthogonal to the direction of movement of the paper or other medium on which printing is to take place through the printer. The printer carriage of a color printer typically has four removable electric or thermal ink jet printheads mounted thereon. Each of the printheads contains a supply of ink which, for large scale printers, is generally inadequate due to the large volumes of ink which are required as compared with the ink supply requirements of desk top printers. Consequently, various mechanisms have been proposed for continuously or periodically refilling the carriage-borne printheads with ink. These systems fall into two categories. The first system comprises offboard or off-axis ink reservoirs that are continuously connected to the carriage-borne or onboard printheads by flexible tubes. An example of the first system is described in U.S. Pat. No. 6,206,512, which is assigned to the assignee of the present application.
The second system comprises a “take a gulp” system in which the printhead carriage is periodically moved to one end of its path of travel where it is then connected with off-axis ink reservoirs to fill the onboard printheads. This “take a gulp” system is disclosed in Hewleft-Packard's Designjet 2000 printer referred to in U.S. patent application Ser. No. 08/805,861 filed Mar. 3, 1997 and published in European Patent Publication No. 0863016 on Sep. 9, 1998.
Such printers include an ink delivery system that provides an ink flow to the printheads from ink reservoirs. Air entrapped in the ink delivery system can cause print defects. To purge or remove the entrapped air, the prior art has used a separate disposable set up pen. This adds cost in the form of extra parts and has a usability disadvantage because the user needs to install and uninstall two sets of pens (set up pens and real printheads), which often is not a well understood process. Another attempt to solve the problem of purging air has used a hydrophobic material in an air vent disposed in a purge chamber. However, when the printer becomes idle between print jobs, the ink is drained from the ink delivery system. This requires that the ink delivery system be purged after each idle time ends before a new print job can be executed. This is a big disadvantage because air purging in large format printers can require several minutes, which is unacceptable to many users.
Thus, the problem of purging air from an ink delivery system without the aforementioned disadvantages remains unsolved.